In a pneumatic tire, what is performed in order to secure driving stability is to bury therein a bead filler extending from a bead portion to a sidewall portion, the bead filler being formed of a rubber composition having a high degree of hardness. Furthermore, there has been proposed an approach to improve a variety of tire performances by devising cross-sectional shapes of the bead filler.
For example, there has been proposed an approach where driving stability and riding comfort are improved without affecting road noise and rolling resistance in a manner that: the bead filler is composed of a wide-width part (a thicker part in a cross section taken along a meridian of the tire) at the side of its inner periphery and a narrow-width part (a thinner part in a cross section taken along a meridian of the tire) at the side of its outer periphery; and the sheet-like narrow-width part is allowed to extend beyond a widest-width position of the tire to a location relatively outward in a radial direction of the tire (refer to Patent Document 1, for example).
However, in such a reinforcement structure of the bead portion as described above, although peripheral stiffness has been considerably increased owing to the sheet-like bead filler extending outward beyond a widest-width position of the tire in a radial direction of the tire, an effect of improving driving stability has not necessarily been sufficient because an effect of increasing a spring constant in a lateral direction of the tire has not been sufficient.
Patent Document 1: Japanese patent application Kokai publication No. Hei11-28916